Monday, September 3, 2012

Build Vs. Buy: The sum of the parts

To those of you in the U.S. Happy Labor Day.  For me it's a great day to spend some time researching my new telescope.

I've now done some thinking about the size of my better scope, and I've decided on something in the 14-16" range.   Large enough to be a big step up from my current scope that can fill my observing needs for long time to come, but small enough to be manageable and affordable.  As I continue to try to scrape together some pennies, I'm left wondering what my options are.

Since I started this project,  I've been leaning heavily towards building my own scope.  Not from scratch, but more by assembling the parts I wanted.  In my mind this seems like a good way for me to control all the various trade offs between price/performance and get just the scope I want, but it also seems that it could end up being a boondoggle of sorts.  I could end up spending more than a commercial scope and getting less.  The tagline on my blog is a joke of sorts, Saving one penny at the cost of two, but I don't want it to be an omen...

To help put my mind at ease, and to get some ideas for the design of my telescope, I've been surveying what is available commercially over the last few weeks.  As luck would have it Matt Wedel, of 10 Minute Astronomy fame, just posted a great overview of telescopes available in the size range I was looking at.  He shares some of my concerns about price and portability, so his pros and cons are instructive for me as well.

With such a wide range of options, and a similarly wide range of prices, how can I go about really knowing which decision, Build Vs. Buy is right?  If I decide to buy, how do I go about choosing which one?  If I decide to build, how can I figure out how much I need to spend?

I think the answer to all of these questions is to look at which basic parts make up a telescope, try to judge their impact on the overall telescope experience and figure out which features are important to me.

Sum of the Parts

I've been around telescopes for a while, and if you are reading this, I bet you have too.  In my observing experiences with my own telescopes and others, each telescope has a different feel... some are easy to use and seem to disappear into the observing experience.  Others assert themselves with distracting issues that interrupt the zen-like flow of observing.

These experiences often center around one or other piece of the scope which is not working as well as I'd like.  I've thought about them in the moment, but never taken an inventory of each part and what I would like to see from them.  This will come in handy as I evaluate commercial scopes and put together a budget and parts list for building a better scope.

Most of these will warrant a post to themselves as I try to figure out the key measures of quality for each and evaluate different options and suppliers.  Here are my main areas of consideration in no particular order:

Mirrors
Size is king, but there are also considerations such as focal length, thermal properties of the material, weight vs. deformation and shape (conical vs. flat).   Each manufacturer seems to have different standards of, and reputations for, quality.  In my experience the mirror is probably the largest factor in the overall experience of a telescope and I've already spent some time trying to figure out where the value/dollar sweet spot is.

Mass produced, or machine produced, mirrors are less expensive but quality can be hit and miss.  Most commercial scopes will have some sort of mass produced mirror.  Hand figured mirrors are usually higher quality, since some person spent time testing and fine tuning them, but can be pretty expensive.

Focuser
Unless I am lucky enough to have a full set of parfocal eyepieces some day, I'm going to need to refocus my telescope.  A cheap, sticky, sloppy or overly fast focuser can be a big annoyance over a night of observing.  With faster focal ratios, the critical focus plane gets thinner and thinner, so a good quality focuser is a must.  A very nice focuser can run upwards of $250 and this is an area I've seen some commercial scopes cut corners.

Motion
This is not really a part, per se, but it's directly dependent on the design and quality of the moving parts of the mount.  It's the feeling one has when pushing the telescope around the sky.  Is it easy to move?  Does it require the same amount of force to move in altitude as it does in azimuth?  Can it be knocked off target by a stray bump of the eyepiece?

I've already decided on a dobsonian style scope, so the basic design will be the same no matter if I build or buy, but there are a lot of different ways to implement the same design.  This is an area where the details matter.  The sizes of the various surfaces that rub against each other, the materials they are made of, and the weight and balance of the overall telescope are all critical to the motion of the scope.

Aside from a few, very high end, commercial scopes, it seems like this is always the first area people try to modify when they purchase one.  I've done a fair amount of tweaking on my XT8 to improve the motion, with some success, but I want my next telescope to be a real joy to use, without a lot of modifications.

Finders
Since I'm not a fan of goto or push-to systems, I spend a lot of time with my finders.  Locating deep sky objects can be a fun challenge, but working with a bad finder is a major frustration.  Aside from the mirror, the is probably the next most important aspect for me.  I like having a unit (non-magnified red dot or reticle) finder for getting the scope in the right part of the sky, and a magnified correct image finder for fainter star hopping.

From what I have seen so far, commercial scopes either come with a unit finder (if you are lucky) or some sort of terribly small straight through finder.  I'm figuring that regardless of my build/buy decision, I'll be purchasing a new finder, so it's sort of a wash.

Spider
The spider is the structure that holds the secondary mirror near the top of the telescope.  It is not a component I really think about, until it comes time to collimate my telescopes.  Having to re-align the secondary is infrequent, so this is not a huge concern, but there are some options that impact the appearance of bright objects.  Since the arms of the  spider are in the light path, they will diffract light and cause scattering and other noticeable optical effects.

I don't think a particular spider design is going to make or break my build vs. buy decision, but some people are passionate about it, so I don't want to ignore this subtlety in a scope I hope to have for a long time.

Mirror Cell
With my largest telescope being 8" I've never had to think about the mirror cell.  It's the thing that holds the mirror and joins it with the rest of the telescope.  At a minimum, it has to keep the mirror from falling out and have some way to adjust it's tilt to get the telescope properly collimated.

An 8" mirror does not require anything that special for support.  The mirror is going to be pretty light, and since it's somewhat small it's not going to flex much under it's own weight.  As mirror size and weight increase, supporting it without changing it's shape becomes a trickier task.  14-16" is where this component starts to matter.  What good is a wonderfully figured mirror if it is deformed under it's own weight as the telescope it moved around the sky?

The mirror cell is not going to be like a bad focuser, reminding you of it's presence all the time, but stability, adjustment, protection, and air flow are all important issues to think about.  They will all affect the overall performance of the scope, and in ways that might not be immediately obvious like a bad focuser.  I've not checked into the commercial scopes I might consider, so I'm not sure how they handle these issues.  If I choose to build, I think this is going to be one of the more difficult parts to research and source.

Structure
The Structure is the skeleton of the scope... the stuff that holds all the other stuff together and keeps it aligned correctly.  It holds everything off the ground and lets the mirrors be pointed at various parts of the sky.  Ideally, it keeps everything aligned perfectly as the direction of gravity changes.  The larger and heavier the mirrors and other components, the more difficult building a good structure becomes.

With smaller scopes, like my XT8, the structure is a solid tube with the mirrors, spider, focuser, finders, etc, all bolted on.  A solid tube is strong, and stiff, but with larger telescopes they can be heavy and overly long.  Larger scopes need something a bit more clever.  There are a lot of different solutions to this, each with their own weight/size/stiffness tradeoffs.

I've seen commercial scopes with very different designs, and even more variety of home built telescopes.  Since size and weight are important considerations for me, and I want a telescope that will last a long time and move well, the structure of the scope will be nearly as important as what it's holding together.



Those are some of my thoughts on the criteria I can use to evaluate different commercial options, and to think about the parts I might need to build one of my own.  I'd love to hear any thoughts you might have on how important you've found each of these, or if I've overlooked some pieces of the puzzle.






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